1. Field of the Invention
This invention relates to processes and apparatus for digesting aqueous organic materials, such as sewage from household and municipal sources, or industrial or trade wastes. It particularly relates to anaerobic digestion thereof at elevated temperatures.
2. Review of the Prior Art
The anaerobic digestion of aqueous organic materials is considered to consist of the reactions occurring in series as illustrated below and as discussed by John F. Andrews in "Control Strategies for the Anaerobic Digestion Process, Part I" in Water and Sewage Works, March, 1975. ##STR1## It may be seen that the anaerobic digestion process actually occurs in two general steps: (1) the acid fermentation stage and (2) the methane fermentation stage.
In the second stage, highly obligate anaerobes, collectively known as methane formers, are required. The presence of any free or dissolved oxygen will destroy or at least severely inhibit further digestion because of the oxygen-sensitivity of the methane formers. A true anaerobic system must not contain any available or free oxygen; chemical oxygen alone is used in the oxidation-reduction reactions. A free oxygen content as low as 0.08 ppm is lethal to many of the true or obligate anaerobes.
The Andrews article (supra) reveals that the acid-producing bacteria are less susceptible than the methane bacteria to changes in environmental conditions such as pH, temperature, and inhibitory substances. Moreover, because most species of methane bacteria have much lower growth rates than the acid producing bacteria, the rate-limiting step in the overall anaerobic digestion process is the conversion by the methane bacteria of the intermediate products (i.e., the volatile acids and carbon dioxide produced by the acid producing bacteria) to methane and carbon dioxide. It is also known that acetic acid is the precursor of approximately 70% of the methane produced in the digestion of organic sludges from municipal waste water treatment plants. The specific growth rate for methane bacteria was shown by Andrews to be significantly inhibited by decreasing pH, particularly with total substrate concentrations beyond about 20 millimoles per liter.
Obviously, methane formers that cannot exist outside of their particular pH, temperature, and true anaerobic condition could not conceivably be present in present-day sludge digestors or extended aeration trickling filters or septic tanks. Sludge digestors that operate at a normally fixed temperature of 24.degree.-28.degree. C clearly exclude most of the essential methane formers and required pH ranges.
United States Pat. No. 1,963,581 teaches the organisms in ordinary sludge digestion are most active at temperatures between 16.degree. C and 20.degree. C and are killed at temperatures of 49.degree. C to 60.degree. C which are best for growth of facultative thermophiles and obligate thermophiles. These thermophiles are also most active in sludge digestion within the pH range of 7.8-8.2. Some of the bacteria in the thermophilic group are liquifying bacteria which provide food from cellulosic material for gasifying bacteria so that it was thought that an insufficiency of liquifying bacteria could cause a shortage of food for the gasifying bacteria. Lime was added to the sludge to produce food that would be directly available to the gasifying bacteria, i.e., organic salts such as calcium acetate, thereby rendering the gasifying bacteria independent of the liquifying bacteria. In addition the sludge was heated to temperatures ranging between 45.degree. C and 70.degree. C to destroy the major portion of the bacteria active at lower temperatures by stimulating the reproduction of and activity of thermophilic bacteria, thereby increasing gas production and greatly shortening the time required by digestion of the sludge.
It is shown in the prior art that anaerobic digestion processes can be doubled in rate for each 10.degree. C rise in temperature between 10.degree. C and 60.degree. C, thereby reducing the time required for digesting from six months at the lower temperature range to 12 days at 60.degree. C. Accordingly, a sludge digestor is described in U.S. Pat. No. 2,430,519 for digestion of sludges and similar highly concentrated wastes in subsequent stages having successively lower temperatures, the initial stage of digestion being at a temperature of about 102.degree.-108.degree. F (38.9.degree.-42.2.degree. C). Another sludge treatment is described in U.S. Pat. No. 2,188,847, wherein sludge is heated at the bottom of a deep tank having a central convection tube.
Although sludge digestors are normally considered to be anaerobic units, they are not treatment plants but are means for considerably reducing high concentrations of organic wastes which are accumulated in treatment plants. The substrate of such digestors have 5-day biological oxygen demands in the order of 20,000 ppm or greater. The more prevalent type of anaerobic unit, which is not properly considered a treatment plant, is the septic tank.
In the United States, census figures show that approximately one-third of the population is served by individual household treatment systems for waste water disposal. In 1970 there were approximately 17 million septic tank systems, and such septic tank soil absorption systems will continue to be used for some time in the future. The coliform count of the effluent from typical septic tanks may be higher than one million per milliliter, a number which is 10,000 times the permissible level of the recommended 1968 standards for surface water used for bathing. Typically, septic tanks remove about 50% of chemical oxygen demand (COD), about 50% of the biochemical oxygen demand (BOD), and about 75% of the suspended solids of the untreated effluent.
Normal household septic tanks are usually cylindrical or rectangular concrete structures of 400 gallon capacity in which household wastes are treated at ambient temperatures only. No attempt is made, other than occasional liming, to control the pH. The effluent is passed through crushed stone filter drains for further biological treatment and absorption. Unfortunately, the interstitial spaces of the drains and soil become clogged with sewage solids, resulting in localized ponding that causes foul odors and health hazards. Such effluent can be expected to be in the range of 12-1,400 ppm of biological oxygen demand over five days (BOD.sub.5), 1,800-2,000 ppm of suspended solids, and 1.times.10.sup.8 /ml..degree. C of ESCHERICHIA coliform organisms.
Recognizing the methane fermentation stage to be the rate limiting step of the anaerobic digestion process, suggestions have been advanced in the art to accelerate the reaction rates of that step. For example, Pohland and Ghosh have proposed a two-phase anaerobic digestion process in which the acid and methane formers were provided with optimum environments in separate acid and methane fermentation reactors connected in series ("Developments in Anaerobic Treatment Processes", Biological Waste Treatment, Wiley Interscience Publishers, New York, New York (1971), at page 85; "Anaerobic Stabilization of Organic Wastes-Two-Phase Concept", Environmental Letters, April, 1971, at page 255). Also, Borchardt described a dialysis technique in which the acid- and-methane fermentations were separated into separate chambers ("Anaerobic Phase Separation by Dialysis Technique", Advanced Chemistry Ser., 105, 108 (1971)). The fastidious methane flora were fed with the products of acid fermentation permeating through a membrane.
Treatment plants presently used for domestic sewage and industrial waste are frequently troubled by foul odors, noise from air-blowers, compressors, pumps, and agitators, and inability to treat underload or overload conditions. Treatment plants for individual households or small groups of households, particularly septic tanks, are not satisfactory because the output therefrom is often ecologically unacceptable. Furthermore, grease traps often used in conjunction with such treatment systems sometimes create obnoxious odors and require frequent cleaning.
Accordingly, there is a need for a sewage treatment plant that is operable without maintenance, without skilled supervision, and without machinery. There is further a need for a sewage treatment plant that can utilize an anaerobic process at elevated temperatures with means for achieving a sequential gradation of temperature. It is further desirable to have an anaerobic process that has a sterilized effluent. It is also important in such an anaerobic process to provide additional means for deodorizing and oxidizing the treated and sterile effluent and then removing the nitrogen so that the fluid can be added to existing waterways without ecological deterioration.